Research On Grid-connected Control Of Permanent Magnet Wind Power Based On The Improved Quasi-Z-source Inverter

With the development of new energy,solar photovoltaic power generation technology and wind power technology as the representative of the new energy power generation technology have obtained rapid development.Among them,the wind energy is popular because of the inexhaustible and pollution-free.Because of the uncertainty and the gap of the wind speed,the amplitude and frequency f the output voltage of the permanent magnet synchronous generator vary with the speed,and can not be directly connected to the grid.Therefore,there is a converter between the generator and the power grid.The grid-connected inverter as the interface device between the generator and the power grid will directly affect the performance of the whole system.Three-phase voltage source inverter is often used in the traditional converter,but the three-phase voltage source inverter is buck inverter,in order to meet the requirements of wind power system require the addition of BOOST type DC-DC conversion circuit,the addition of circuit reduces the efficiency of the system to a certain extent,and makes the converter device into a secondary structure,so that the control is more complicated.In order to prevent the switch device from being turned on or off,the dead time is added to the switch signal,but the increase of the dead time will increase the harmonic content of the output waveform.As a new type of power converter topology,the Z-source inverter overcomes the disadvantages of the traditional inverter,it is no need to insert dead time and has the boost characteristic.The main advantage of the Z-source inverter is that the system can produce a high AC output voltage with a single stage converter under a low DC input voltage.Z-source inverter also has some defects,such as low boost factor,large capacitance voltage stress.Aiming at the defects of the traditional Z-source inverter,based on the traditional Q uasi-Z-source inverter,three kinds of improved Q uasi-Z-source inverter topologies are proposed to improve the performance of the inverter.Finally,it is applied to the grid connected wind power generation system,and it can realize the effective grid connection by the double closed loop control strategy.The main contents of this thesis are as follows:(1)Firstly,the working principle and operating characteristics of the permanent magnet direct drive wind turbine are analyzed in detail,and the mathematical model of the wind turbine is established.(2)Aiming at the defects of the traditional Z-source inverter,three kinds of improved Quasi-Z source inverter topologies are proposed,by exchanging the switched inductor unit with the inductive device in the traditional Q uasi-Z-source inverter.Using the the change of the working state of the diode in the switched inductor,the energy stored in the switched inductor is transferred to the load during the shoot-through state,thereby enhancing the boost capability of the inverter.Through the analysis and verification,compared with the traditional Z-source inverter,the improved Quasi-Z-source inverter can achieve a higher voltage gain,the impedance of the network capacito r voltage stress is effectively reduced and there is no starting shock current.(3)For the DC side,the small signal model is established by using the state space averaging method.According to the small signal model,the control strategy of the voltage loop is constructed,and the DC link voltage is controlled indirectly by the capacitor voltage.For the AC side,the vector control based on the grid voltage orientation is adopted,the vector control switch frequency is fixed,and the control effect is good,so it is widely used.Finally,the control strategy of the capacitor voltage loop of the impedance network and the current loop of the network side network is selected.